In the related art, in general, a copper plate consists of pure copper used in the above-described copper processed products is produced by a forging process of producing a pure copper ingot, a hot working process of performing hot working (hot rolling or hot forging) on the ingot, a cold working process of performing cold working (cold rolling or cold forging), and a heat treatment process of performing heat treatment for refining crystal grains or reducing distortion. In addition, the cold working process and the heat treatment process may be repeated as necessary.
By cutting the copper plate using a milling cutter, a drill, or the like or performing plastic working such as bending on the copper plate, a product having a desired shape is processed. Here, in the above-described copper plate, in order to prevent gouges and deformation during processing, it is necessary to refine crystal grain diameters and decrease residual distortion.
In addition, in recent years, the above-described copper plate is used as a sputtering target for a wring material of a semiconductor element. As the sputtering target, a copper plate having high purity in which impurity content is limited to decrease electric resistance of a wire on which a film is formed is used. In addition, in order to form a uniform sputtering film, a copper plate in which variations in crystal grain diameters and crystal orientation decrease is required.
In the related art, for example, in order to obtain the copper plate used in the sputtering target or the like, methods disclosed in PTLs 1 to 3 are suggested.
PTL 1 discloses a method for obtaining a sputtering target which has a substantially recrystallized structure, an average crystal grain diameter of 80 microns or less, and a Vickers hardness of 100 or less by performing hot working on a pure copper ingot having a purity of 99.995 wt % or greater, annealing the ingot, which has been subjected to the hot working, at a temperature of 900° C. or less, performing cold working on the annealed ingot at a rolling ratio of 40% or greater, and thereafter, performing recrystallization annealing on the ingot, which has been subjected to the cold working, at a temperature of 500° C. or less.
In addition, PTL 2 discloses a method for obtaining a sputtering target in which the content of copper in a state where gas components are removed is 99.999% or greater, an average particle diameter on a sputtering surface is 250 μm or less, variations in average particle diameters are within ±20%, an X-ray diffraction intensity ratio on I(111)/I(200) sputtering surface is 2.4 or greater, and variations in the X-ray diffraction intensity ratios are within ±20%, by performing hot working such as hot forging or hot rolling on a copper ingot having high purity of 6N or greater at a reduction ratio of 50% or greater, performing cold working such as cold rolling or cold forging on the ingot, which has been subjected to the hot working, at a reduction ratio of 30% or greater, and thereafter, performing heat treatment on the ingot, which has been subjected to the cold working, at 350° C. to 500° C. for 1 to 2 hours.
In addition, PTL 3 discloses a sputtering target which is obtained by removing a surface layer of an ingot formed of a copper having high purity of 6N or greater and additional elements, and thereafter, by performing hot forging, hot rolling, cold rolling, and heat treatment process on the ingot. Particularly, in an embodiment of PTL 3, it is described that a surface layer of a produced ingot is removed to form an ingot of φ160×60t, hot forging is performed at 400° C. to form an ingot of φ200, hot rolling is performed at 400° C. to form an ingot of φ270×20t, cold rolling is performed to form an ingot of φ360×10t, heat treatment is performed on the ingot at 500° C. for one hour, the entire target is cooled, and a target material is formed.
In a production method of a copper plate in the related art which is representative of a production method of a sputtering target, a crystal grain diameter, orientation of crystals, or the like is controlled by performing hot working (hot forging or hot rolling) on a pure copper ingot, and thereafter, performing cold working (cold forging and cold rolling) and heat treatment on the ingot which has been subjected to the hot working. In addition, since cold working conditions and heat treatment conditions greatly influence the crystal grain diameter, the orientation of crystals, or the like, in PTLs 1 to 3, the cold working conditions and the heat treatment conditions are defined.